How many possible distinct arrangements are there of the letters in the word DOG?
We could list them: DOG, DGO, ODG, OGD, GDO, GOD.  Or we could have noticed that we have 3 ways to pick the first letter, 2 ways left to pick the second, and 1 way to pick the third, for a total of $3\cdot2\cdot 1 = \boxed{6}$ ways.